Question: Which of these numbers is prime? ${10,\ 45,\ 57,\ 73,\ 87}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 10 are 1, 2, 5, and 10. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 57 are 1, 3, 19, and 57. The factors of 73 are 1 and 73. The factors of 87 are 1, 3, 29, and 87. Thus, 73 is a prime number.